1. Field of the Invention
This invention relates to a method and apparatus for determining the Btu value of fuel gases. More particularly, this invention relates to a method and apparatus for determining the BTU value of pipeline gas in real time.
2. Description of Related Art
Accurate measurement of the heating value, i.e. BTU content, of hydrocarbon fuel gases is an essential component of their efficient and cost-effective use in a wide variety of applications. The heating value of natural gas, which typically contains in the range of about 80% to nearly 100% by weight methane, generally varies between about 900 and 1300 BTU/ft3 depending upon the composition of the natural gas. In addition to methane, natural gas may contain various amounts of other hydrocarbons, such as ethane, propane, butane and non-hydrocarbons, such as CO2 and N2. The presence of such other components, in addition to affecting the heating value of the natural gas, may also affect the applications in which it is used, particularly applications which are sensitive to heat input. Because the heating value of natural gas consumed is the basis upon which consumers are charged, accurate measurement of the heating value on a continuous basis is essential to ensuring that consumers are neither undercharged nor overcharged for the amount of natural gas consumed. In addition, accurate measurement of the heating value on a continuous basis is essential to ensuring delivery of a gas having a constant BTU content to the consumer. To meet these requirements, monitoring of the heating value is required during all aspects of transmission from the source, storage and distribution to the consumer.
Currently, gas chromatography, calorimetry, NMR and absorption spectroscopy are typically employed for this purpose. However, these methods typically employ bulky or expensive equipment and are less than ideal for in-situ measurements. Also used are gas sensors comprising a coil of a fine platinum wire coated with a catalyst to form a bead. During operation, the sensor is heated by passing a current through the platinum wire whereby, when a combustible gas contacts the hot catalyst on the bead surface, the hydrocarbon gas reacts to produce heat from which the heat content of the gas can be determined (See, for example, U.S. Pat. No. 5,012,432 and U.S. Pat. No. 5,494,826). However, at least some of these devices are susceptible to numerous sources of error such as the composition of the sample gas and the age and composition of the sensor being utilized.
One recently developed approach to BTU measurement is an inferential natural gas flow rate system, which is based on sonic speed measurements and a cross correlation method used to compare two reflected signals to yield their time delays in a manner independent of the reflector distance from the transducer. However, in many instances, tests on a low pressure loop yielded errors from 1 to 4% in the sonic speed measurements. Thus, there is a need for a device that is consistent, accurate to better than 1%, and reliable.
At the present time, the energy content or heating value of pipeline gas, i.e. natural gas, is measured only at gas custody transfer areas. Because pipelines run several hundred miles after the transfer custody points, the energy content could be different at the distribution networks. Thus, it is necessary that the energy content at these locations be measured. Accordingly, a device having the desired accuracy and consistency, which can be used in-situ, and which can provide real-time heating values is particularly desirable to large individual customers having distribution network applications and to large industrial consumers of pipeline gas.